Non-aqueous silver film formation

ABSTRACT

An anhydrous silver plating solution comprising an alcohol such as methanol or ethanol; a silver salt such as silver nitrate; a hydroxide such as NaOH, KOH, etc.; and a complexing agent such as ammonia, ethylene diamine, or pyridine. Addition of an anhydrous reducing aldehyde such as dextrose to the solution results in a thin silver plating on objects placed in the solution. The solution is particularly suitable for silver plating electrically nonconductive, water sensitive substances having relatively high vapor pressures.

United States Patent [191 Henriksen [45] Aug. 20, 1974 NON-AQUEOUSSILVER FILM FORMATION [75] Inventor: Gary L. Henriksen, Crofton, Md.

[73] Assignee: The United States of America as represented by theSecretary of the Navy, Washington, DC.

[22] Filed: June 18, 1973 [21] Appl.No.:370,907

[52] US. Cl. 106/1, 117/35 S, 117/130 E [51] Int. Cl. C23c 3/02 [58]Field of Search 106/1; 117/35 S, 130 E [56] References Cited UNITEDSTATES PATENTS 2,879,175 3/1959 Umblia et al. 106/1 X 3,383,247 5/1968Adlhart et al 106/1 X 3,547,818 12/1970 Wade 106/1 X PrimaryExaminer-Lewis T. Jacobs Attorney, Agent, or FirmR. S. Sciascia; J. A.Cooke; M. G. Berger [57] ABSTRACT An anhydrous silver plating solutioncomprising an alcohol such as methanol or ethanol; a silver'salt such assilver nitrate; a hydroxide such as NaOH, KOH, etc.; and a complexingagent such as ammonia, ethylene diamine, or pyridine.

Addition of an anhydrous reducing aldehyde such as dextrose to thesolution results in a thin silver plating on objects placed in thesolution. The solution is particularly suitable for silver platingelectrically nonconductive, water sensitive substances having relativelyhigh vapor pressures.

12 Claims, No Drawings 1 NON-AQUEOUS SILVER FILM FORMATION BACKGROUND OFTHE INVENTION This invention relates generally to coating processes andmore particularly to the formation of silver films by 5 chemicalreduction in solution.

Present methods of silver film formation are chemical reduction aqueoussolution, electrochemical plating, and vacuum deposition. Chemicalreduction in aqueous solution involves the reduction of silver (l) froma soluble complex [e.g., Ag(NH to silver metal. This method has theobvious disadvantage of causing the deterioration of water sensitivematerials. The second method, electrochemical plating, uses an aqueousbath containing salts, including silver salts (e.g., silver cyanide), ananode made of high purity silver, and the object to be plated as thecathode. Obviously, this method will also cause the deterioration ofwater sensitive materials. In addition, this method has the disadvantageof being limited to materials which are electrically conductive or arecoated with electrically conductive materials. The third method, vacuumdeposition of silver, involves evacuating a vacuum chamber containingthe material to be coated to an absolute pressure of about 10' to 10 mmof mercury while heating the silver to its evaporation temperature.While this method may be used for silver coating electricallynonconductive water sensitive materials, it has the disadvantage ofbeing limited to coating materials with low vapor pressures.

SUMMARY OF THE INVENTION Accordingly one object of this invention is toprovide a method for silver plating materials.

Another object of this invention is to provide a method for silverplating water sensitive materials.

Yet another object of this invention is to provide a method for silverplating materials with high vapor pressures.

Still another object of this invention is to provide a method forforming thin silver coatings on water sensitive materials without usinga vacuum system.

Yet a further object of this invention is to make water sensitive,electrically nonconductive materials suitable for electrochemical silverplating in nonaqueous solutions.

These and other objects of this invention are accomplished by providingan anhydrous silver plating bath formed by dissolving a silver salt,such as silver nitrate, in an alcohol such as ethanol, methanol, ormixtures thereof, then adding hydroxyl ions to precipitate the silverout as Ag O, and finally adding a complexing agent selected fromammonia, ethylene diamine, pyridine, or mixtures of these compounds toredissolve the silver as a complex ion. Addition of an anhydrousreducing aldehyde, such as dextrose, to this bath reduces the silver,resulting in the silver plating out on all surfaces in contact with thebath.

DETAILED DESCRIPTION OF THE PREFERRED 60 EMBODIMENT The components ofthe plating bath must be compatible with the materials to be plated.Since the bath is primarily designed for silver plating water sensitivematerial, the components of the bath must be anhydrous and must notinteract to form significant amounts of water. For instance, the solventused should be either absolute methanol, ethanol, of mixtures thereof.The hydroxyl ion source must be anhydrous, preferably dried LiOH, NaOH,KOH, RbOH, CsOH, or mixtures thereof. Hydroxides which contain water ofcrystalization (e.g. Ba- (OH) '8H O, Sr(OH) -8H O) should be avoided.Although the alcohol and hydroxyl ions do react to form some water,

ROH OH 3 R0 H O the equilibrium greatly favors the unionized alcohol sothat very little water is formed. Finally the remaining ingredients, thesilver salts (e.g., AgNO and complexing agent (ammonia, ethylenediamine, or pyridine) must also be anhydrous. The silver ions do reactwith the hydroxide ions to form some water 2 Ag+ H20.

However, this water is consumed during the complexing step (pyridine)Thus, the water formed will not affect the material to be platedprovided that the object to be plated is not placed in the solutionuntil after all the silver has been complexed.

Moreover, the components of the baths should be ested for compatibilitywith the material to be plated. For example, RbAg I is not compatiblewith absolute methanol but it is compatible with absolute ethanol.Simple tests for compatibility will be known to one of ordinary skill inthe art.

Factors to consider in selecting a silver salt are solubility,ionization, and the possible interference of the anion with theprecipitating, complexing, or reducing of the silver. Although any saltwhich will dissolve in at least trace amounts can be used in the presentinvention, preferably the salt should be at least slightly soluble inalcohol. The preferred salt is silver nitrate. Besides solubility inalcohol, silver nitrate offers the advantage of having an anion whichwill not interfere with the precipitating, complexing or plating steps.

Similarly, the hydroxyl ion source should be soluble and ionize inabsolute alcohol and have cations which will not interfere with theprecipitating, complexing, or

.plating of the silver. Solubility and ionization of the hydroxyl ionsource are important because the silver ion complexes are more stableand easily formed at higher pHs. The pH of the solution should beadjusted to at least 8 but preferably more than 11. The preferredhydroxyl ion sources are LiOH, NaOH, KOH, RbOH and CsOH, with NaOH andKOH being the most preferred. Moreover, a saturated solution of thehydroxyl ion source in alcohol is preferable.

The complexing agents are selected on the basis of their relativeholding power on silver ions in absolute alcohol. For the threecomplexing agents used in this invention, the order of increasingholding power is pyridine, ammonia, and ethylene diamine. Pyridine, theweakest, will not form a strong enough complex with silver ion inethanol to work. However, pyridine does work in methanol. On the otherhand, the silver (I) ethylene diamine complex, [Ag(en) wherein en isethylene diamine and n l, is very stable in ethanol at room temperature.The silver (I) is tied up so well that the redox reaction does notobservably proceed when the reducing aldehyde (dextrose) is added untilthe temperature is raised to C.

The amount of complexing agent used determines the amount of silvercomplex that will be in solution available for plating. If any amount ofcomplexing agent is added, some silver complex ions will be formed andwill be available for at least some plating. Preferably enoughcomplexing agent should be added to dissolve all of the silver ascomplex ions. Although each silver ion complexes with two molecules ofpyridine or ammonia, or with one molecule of ethylene diamine, thecomplexes are unstable enough to prefer a large excess of the complexingagent.

For instance, a 90 to 1 molar ratio of pyridine to silver was used inexample lb. The amount of ethylene diamine preferred was about 1/20 ofthe pyridine used. It should be noted that although a 90 to 1 ratio wasused in example lb to complex the silver, an even larger excess of thecomplexing agent may be used with the same results. Thus a molarconcentration greater than zero will be operable, but a molarconcentration of from more than zero to 90 times the molarsaturationconcentration of the silver salt used is preferred. In the case ofammonia, or ethylene diamine a molar concentration of from more thanzero to ten times the molar saturation concentration of the silver saltused is even more preferred.

Any anhydrous reducing aldehyde should work in this invention. Dextrosewas used in the examples. Note that any molar concentration of reducingaldehyde greater than zero will reduce some silver and cause it to plateout; however, a molar amount of at least equal to and preferably twicethe molar saturation concentration of the silver salt should be used tomaximize the amount of silver reduced.

A number of factors should be considered in determining the amount ofreduced silver needed to coat the water sensitive material. First, thesilver plates out onto all surfaces in contact with the solution,including the inside walls of the container holding the plating bath.Second, the method provides only a thin coating of silver on thesurfaces after which the excess reduced silver forms granules which fallto the bottom of the bath. Thus by adjusting the silver concentration tothe point where no or very small amounts of granules are formed, theoptimum silver concentration can easily be found. Also, although thesilver coating provided by this method is thin, it is protective andelectrically conductive. Thus, nonaqueous electrochemical means can beused to thicken the coating of silver. Moreover, since the silver stopscoating on a surface after a given thick ness, once the inside walls ofthe container containing the bath are coated they no longer interferewith the coating process in future batches.

The general nature of the invention having been set forth, the followingexamples are presented as specific illustrations thereof. It will beunderstood that the invention is not limited to these specific examplesbut is susceptible to various modifications that will be recognized byone of ordinary skill in the art.

EXAMPLE 1 Standard solutions of the following concentrations in absolutemethanol were prepared:

Silver Nitrate 0.097 X 10 moles/cc Sodium Hydroxide 0.124 X l moles/ccDextrose 0.l 12 X moles/cc EXAMPLE IA 1.50 cc of the sodium hydroxidesolution was added to 5.0 cc of the silver nitrate solution, forming abrown precipitate of Ag O. Then 2.50 cc of the pyridine was addeddissolving the Ag O precipitate. Finally, 1.50 cc of the dextrosesolution was added, reducing the silver and causing a silver plating toform on the walls of the test tube.

EXAMPLE 1B The experiment of example 1A was repeated using the followingproportion of the above solutions: silver nitrate, 5.00 cc; sodiumhydroxide, 2.25 cc; pyridine, 3.50 cc; and dextrose, 2.50 cc. A thinsilver coating was formed on the sides of the test tube and the excessreduced silver formed granules.

EXAMPLE IC The experiment of example IB was repeated using the followingproportions of the above solutions: silver ni trate, 5.00 cc; sodiumhydroxide, 2.00 cc; pyridine, 3.00 cc; and dextrose, 4.00 cc. Again athin silver coating was formed on the sides of the test tube and theexcess reduced silver formed granules.

EXAMPLE ll EXAMPLE III The standard solution of silver nitrate, sodiumhydroxide, and dextrose used in example I were used in the example.

EXAMPLE IlIA 1.50 cc of the sodium hydroxide solution was added to 5.00cc of the silver nitrate solution, forming a brown precipitate of Ag O.The solution was stirred while ethylene diamine was added drop by dropuntil all of the Ag O precipitate had dissolved. Finally, 1.50 cc of thedextrose solution was added, reducing the silver and causing a silverplating to form on the walls of the test tube.

EXAMPLE lIlB 2.25 cc of the sodium hydroxide solution was added to 5.00cc of the silver nitrate solution, forming a brown precipitate of Ag O.The solution was stirred while ethylene diamine was added drop by dropuntil all of the Ag O precipitate had dissolved. Finally, 1.50 cc of thedextrose solution was added, reducing the silver and causing a silverplating to form on the walls of the test tube.

EXAMPLE lIIC 2.00 cc of the sodium hydroxide solution was added to 5.00cc of the silver nitrate solution, forming a brown precipitate of Ag O.The solution was stirred while ethylene diamine was added drop by dropuntil all of the Ag O precipitate had dissolved..Fina1ly, 4.00 cc of thedextrose solution was added, again causing the silver to reduce and forma thin coating on the walls of the test tube.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. An anhydrous silver plating bath comprising:

a. an alcohol selected from the group consisting of methanol, ethanol,and mixtures thereof;

b. a silver salt in a molar concentration greater than zero to asaturated solution;

c. a hydroxyl ion source in an amount sufficient to adjust the pH to avalue greater than 8;

d. a complexing agent selected from the group consisting of ammonia,ethylene diamine, pyridine, and mixtures thereof in a molarconcentration of greater than zero;

provided that when said complexing agent is pyridine said alcohol ismethanol.

2. The anhydrous silver plating bath of claim 1 wherein said silver saltis silver nitrate.

3. The anhydrous silver plating bath of claim 1 wherein the hydroxyl ionsource is in an amount great enough to adjust the pH to a value greaterthan 11.

4. The anhydrous silver plating bath of claim 1 wherein said hydroxylion source is selected from the group consisting of LiOH, NaOH, KOH,RbOH, CsOl-l,

and mixtures thereof.

5. The anhydrous silver plating bath of claim 4 wherein said hydroxylion source is selected from the group consisting of NaOH, KOH, andmixtures thereof.

6. The anhydrous silver plating bath of claim 1 wherein said complexingagent is in a molar concentration of greater than zero to times themolar saturation concentration of said silver salt.

7. The anhydrous silver plating bath of claim 6 wherein said alcohol ismethanol and said complexing agent is pyridine.

8. The anhydrous silver plating bath of claim 6 wherein said complexingagent is selected from the group consisting of ammonia, ethylenediamine, and mixtures thereof.

9. The anhydrous silver plating bath of claim 8 wherein said complexingagent is in a molar concentration of from more than zero to 10 times themolar saturation concentration of said silver salt.

10. A method of plating a surface with silver comprismg:

contacting said surface with the anhydrous silver plating bath of claim1; and

adding an anhydrous reducing aldehyde to said anhydrous silver platingbath in a molar amount greater than zero.

11. The method of plating a surface with silver of claim 10 wherein saidreducing aldehyde is added in a molar amount of greater than zero totwice the saturation concentration of the silver salt.

12. A method as in claim 10 wherein the anhydrous reducing aldehyde isdextrose.

2. The anhydrous silver plating bath of claim 1 wherein said silver saltis silver nitrate.
 3. The anhydrous silver plating bath of claim 1wherein the hydroxyl ion source is in an amount great enough to adjustthe pH to a value greater than
 11. 4. The anhydrous silver plating bathof claim 1 wherein said hydroxyl ion source is selected from the groupconsisting of LiOH, NaOH, KOH, RbOH, CsOH, and mixtures thereof.
 5. Theanhydrous silver plating bath of claim 4 wherein said hydroxyl ionsource is selected from the group consisting of NaOH, KOH, and mixturesthereof.
 6. The anhydrous silver plating bath of claim 1 wherein saidcomplexing agent is in a molar concentration of greater than zero to 90times the molar saturation concentration of said silver salt.
 7. Theanhydrous silver plating bath of claim 6 wherein said alcohol ismethanol and said complexing agent is pyridine.
 8. The anhydrous silverplating bath of claim 6 wherein said complexing agent is selected fromthe group consisting of ammonia, ethylene diamine, and mixtures thereof.9. The anhydrous silver plating bath of claim 8 wherein said complexingagent is in a molar concentration of from more than zero to 10 times themolar saturation concentration of said silver salt.
 10. A method ofplating a surface with silver comprising: contacting said surface withthe anhydrous silver plating bath of claim 1; and adding an anhydrousreducing aldehyde to said anhydrous silver plating bath in a molaramount greater than zero.
 11. The method of plating a surface withsilver of claim 10 wherein said reducing aldehyde is added in a molaramount of greater than zero to twice the saturation concentration of thesilver salt.
 12. A method as in claim 10 wherein the anhydrous reducingaldehyde is dextrose.